lost channel

[pjnestor]

Hi,

I finished a stereomour about 2 weeks ago.  It initially worked fine, but after a few days, the right channel stopped.  Switching tubes made no difference.  Recheck of voltages shows 0 at terminal 5, A1 and A4 (all should be 60)  Alll the other voltages appear correct.  Any idea what to check next?

Pat

[Paul Birkeland]

Hello Pat,

Way to go on nailing things down really well!

If the 0 at 5, A1, and A4 stays regardless of which 2A3 is in the socket, then you have things well narrowed down.  I'd also guess that the voltage at A2 is a little higher than on the working channel.

I'd begin looking at the combination of the 1.6K and 4.7K resistors on the offending channel.  Look at where both ends connect and be sure the solder joints are good, then check the joints leading up to the hum pot and to the 4 pin socket.  Somewhere within those joints, there's probably a funky solder joint that has worked loose. 

-PB

[pjnestor]

Thanks for the quick reply.  I'm not sure which was the problem but reflowed those connections and its working again.

Pat

[pjnestor]

Well, perhaps I posted too soon.  The right channel is working, though not nearly as loudly as the left.  Voltages which were previously 0 are now 60, as they are supposed to be.  Any idea where I should look next?

Thanks,
Pat

[Paul Birkeland]

Hello Pat,

I'd recommend temporarily connecting a wire between pin 3 on each 4 pin socket.  This will tell you if the output stage is operating correctly (both channels will get fed the same signal, so they should be equally loud).

Let me know how this test goes, I can provide some very specific advice based on these results.

-PB

[pjnestor]

Paul,

Connected A3 to C3.  Still lower volume on the A side.

Pat

[Paul Birkeland]

OK, good news, that narrows things way, way down!

Check the solder joints around the 3.3uf cap, the plate choke leads, and especially all the joints on the output transformer.  Also, the joints on the speaker posts can be a bit tought to get soldered all the way.

-PB

[pjnestor]

Paul,

Thanks for all your suggestions and help.  I resoldered all those connections twice but nothing has changed.  Is there something else I could have missed?

Pat

[Paul Birkeland]

Hello Pat,

OK, you can connect pin A2 to pin C2, then test the output levels again.

This will tell you if there is an issue with the wiring around the output transformer, or if it's behind the output transformer (plate choke, power supply, etc.).

There's also the slightest possibility that the 220uf cathode bypass cap on your offending channel isn't well connected to the circuit (does your weak channel have especially weak bass?).  These caps are on the big power supply PC board, and they are the ones up front by themselves. You should be able to remove the nuts that hold the board down, tilt it up, then reflow these joints just to be sure.  (Maybe try this before doing the test connecting A2 and C2)
 
Thanks for all your patience, we'll get this nailed down.

-PB

(Grainger - the 60V is the cathode bias voltage on the 2A3).

[pjnestor]

Paul,

I think that step localized the problem.  As I was taking off the circuit board to check the 220uf capacitors, the 5w 360 ohm resistor next to it cracked with blackened circuit board underneath.  In retrospect, I can see that the edge of the board was darkened.  I probably should have seen it sooner (a learning experience, right?)  Could that have caused the lower volume?  How do I go about fixing it?

Pat

[Paul Birkeland]

Yes, that would have caused lower voltage at pin 2 of the 4 pin socket.

PM me to confirm your name and I'll mail you a replacement tomorrow.

-Paul

[Paul Birkeland]

Hello Pat,

Parts are on the way today! 

If you plan to make it to the Seattle area any time in the near future, I'd be happy to look it over for you.

Sincerely,
Paul

[pjnestor]

Thanks, that's great Paul.  I have to go down to Bellevue next week anyway.  Hopefully, replacing the resistor will fix it, but if not, I may stop by.  Maybe get a chance to check out the 300b pre too.

Pat

[pjnestor]

Paul,

I got the new resistor and replaced the other one, but with no change in the sound.  Would it be OK to bring it in on Wednesday afternoon.  I'll be in Seattle anyway.

Thanks,
Pat

[pjnestor]

Thanks for all your help, Paul.  It's sounding great now.